The American Civil Liberties Union (ACLU) on Thursday said President Trump's plan to declare a national emergency to build a wall along the southern border would be a "clear abuse of presidential power."

"President Trump's hankering for a wall at the southern border cannot be justified by calling a national emergency. This would be a clear abuse of presidential power - one that sidesteps the role of Congress in the appropriation of funds," ACLU executive director Anthony Romero said in a statement.

"Shame on any member of Congress who doesn't clearly and vigorously speak out on this illegitimate invocation of emergency authorities," Romero added. "The chickens will come to roost when the next president uses these powers to call a national emergency on gun control or climate change."

The White House said Thursday that President Trump would sign a bill to reopen the government and avoid another shutdown, but added that he would separately declare a national emergency to get funding for a border wall, which he has long demanded.

Trump for weeks has signaled that he would declare a national emergency if Congress didn't approve $5.7 billion in funding for the wall. Democratic lawmakers, as well as some Republicans, have predicted that such a declaration would bring an immediate legal challenge.